Air carrier flight crew reports taxiway excursion during turn into the ramp; due to icy conditions and strong cross wind.

Narrative

Freezing rain was falling in ZZZ while we were enroute; with visibility 9-10 miles and at +/-1 degree above freezing. We kept an eye on the ATIS and noted that braking action was reported as fair on all runways (reported by vehicle); with Runway XXR slightly better with a Mu reading of 42. Our Dispatcher sent a message advising that Runway XXR braking action was good; but that ramp braking action was poor to nil. With this in mind; I briefed my First Officer (Pilot Flying) that we'd use both engines for taxi after landing; and if it looked too slick we'd stop and request a tow to the gate. An RJ landed ahead of us on Runway XXL; reporting braking action good. My First Officer used medium autobrakes as a precaution; but we concurred with the good braking action and had no trouble exiting the runway. I began our northbound taxi on Taxiway T; and when we contacted Ground; the Controller again stated braking action was good per an RJ. I could see all taxiway surfaces were at least wet; as it was raining at this time. I kept my taxi speed at a slow rate as we approached the short stub taxiway that runs from Taxiway T across Taxiway S and into the Ramp. Though I thought I was being conservative; as I eased into the 90-degree left turn; I immediately sensed the nose was not responding fully to commands. I applied brakes which had no apparent effect. There was no cycling of Antiskid due to the low speed; so I assume the mains were sliding at this point. I tried modulating the brakes; and with full left steering wheel input the nose slowly responded by moving in a wider than desired arc; while remaining on the taxiway through 90 degrees. I believe the near 20-KT south wind and downward slope of the taxiway played a part by causing the tail to weathervane in a slow counterclockwise rotation. There was absolutely no traction whether on or off the brakes. For brief seconds we were pointed in the intended westerly direction; but wind; grade; and ice conspired to point the nose southward as we slid down to the west. Running short on ideas; I had attempted to bring both engine reversers up to stop our forward progress the moment our nose swung to the west. But in the seconds it took for the reversers to respond I could see they would be of little help as the tail was swinging us on through to the southern heading. Only the left reverser had begun to respond and I quickly stowed it when it became evident that it would not stop our lateral slide. We came to rest heading 200 degrees with the right main tires approximately 4 FT off the pavement on the west side of Taxiway S. I asked the passengers to remain seated while we assessed the situation. Numerous vehicles responded quickly; and personnel had difficulty walking as they moved about the aircraft. Trucks laid sand and deicing sprayed around the aircraft so a tug could attempt to push us backward and fully onto the pavement. Despite their best efforts; the tug only spun its wheels on the ice and was unable to move us. 2 busses had been brought to the scene by this time; and we resorted to off-loading the passengers down the ventral stairs. While passengers were deplaning; I called the Duty Pilot and was linked to the Chief Pilot and Dispatcher to make a verbal report. Maintenance personnel advised me that no damage was incurred to the aircraft or taxi lights or signage. An on-site representative informed me that all passengers made their connections after safely deplaning. I had briefed and prepared for challenging conditions on our arrival. I was pleasantly surprised at the condition of the runway and Taxiway T; and despite further encouragement from Ground Control that braking action was good; I expected to encounter ice in and around the Ramp from the Dispatcher's earlier warning. The runway had been deiced and sanded; and I assume Taxiway T had been given some type of treatment. It is apparent tht the taxiway from Taxiway T to Spot X had received no such treatment; and its icy surface and gradient made passage impossible at what I thought was a safe taxi speed. Supplemental information from ACN 818065: What was learned and how to prevent this in the future: It would seem to me that when a runway is treated and then reopened; aircraft should be able to exit that runway safely. That means if the runways are treated; all taxiways and areas leading to and from those runways should be treated as well (or NOTAMED out). Using treated taxi routes; much like low visibility SMGCS charts; would give flight crews assurance that condition of these surfaces had been verified and deemed safe for use. Also; PIREPS; while helpful; may not give a complete picture of what to expect. In this case; I believe the reporting RJ entered the ramp at a different spot. That spot may have been treated or may not have been as contaminated as the surface which we encountered.